1. Field of the Invention
This invention relates to antennas, particularly a GPS antenna operative to null or suppress reception of interfering signals.
2. Discussion of the Known Art
The Global Positioning System (GPS) is a navigation system featuring a constellation of 24 satellites that traverse six circular orbits around the Earth twice a day, with four satellites in each orbit. The satellites transmit coded L-band microwave radio frequency (RF) signals, and are always positioned so that signals from at least four of the satellites can be received at any point on Earth.
Specifically, each GPS satellite presently transmits its signals using two RF carrier frequencies, viz., 1575.42 MHZ (referred to as “L1”) and 1227.60 MHZ (“L2”). A third frequency of 1176.45 MHZ (“L5”) has also been allocated for civilian use in 2009. The L1 signal is modulated by two different spread spectrum codes, namely, an unclassified coarse acquisition (C/A) code intended for use in commercial civil navigation equipment, and a Y code that is modulated on the L1 carrier in quadrature with the C/A code and with half the power of the C/A code. The Y code is a product of an unclassified P (precise) code, and a classified W code. The C/A and the P codes are unique for each satellite. The satellite antennas are configured so that the GPS signals have a right-hand circular polarization (RHCP) as they propagate toward the Earth.
Basically, a GPS receiver compares time data as encoded on a signal received from a given satellite, with a local time at which the signal was received. The time difference is then used to calculate the distance between the receiver and the transmitting satellite. Using calculated distances from at least three satellites, the receiver determines its position on Earth by known triangulation techniques. With four or more satellites in view, the receiver can determine its altitude as well. Once the receiver's position is determined, it may calculate other information such as speed, bearing, and distance to a given destination.
The transmitted signals are subject to degradation by several factors including, inter alia, signal multipath which occurs when a GPS signal reflects off of objects such as buildings or other tall surfaces before reaching the receiver. The calculated distance to the transmitting satellite based on a reflected GPS signal will be greater than the actual line of sight distance, thus causing error in the position determined by the receiver. Other degrading factors may include receiver clock errors, orbital (ephemeris) errors, number of satellites visible (the fewer satellites in view, the lower the accuracy), and the relative positions of the satellites at any given time (accuracy degrades when the satellites are spaced angularly close to one another as seen from the receiver). In a tactical or military environment, intentional jamming is another factor that must be overcome to maintain GPS receiver accuracy.
Differential GPS (DGPS) is a system that operates to correct errors which may exist in GPS signals as they are being received in a certain geographical area, wherein a base station in the area transmits certain correction data to other GPS receivers in the area which, in turn, use the data to increase the accuracy of position and other information they derive from the GPS signals. DGPS has been adopted to assist in the landing of aircraft on ground airport runways as well as on decks of aircraft carriers. For example, U.S. Pat. No. 5,361,212 (Nov. 1, 1994) discloses a DGPS landing assistance system wherein a ground station in the vicinity of a landing strip includes a GPS receiver, and a transmitter for sending data to an approaching aircraft over a wireless data link. The transmitted data includes GPS correction data and a global position of two points that define a desired aircraft glide path associated with the landing strip. Equipment aboard the aircraft includes a GPS receiver, a separate data link receiver for receiving the correction data and the global position of the points defining the glide path, and a computer for determining a corrected global position of the aircraft along with lateral and vertical deviations of the aircraft from the glide path. See also, U.S. Pat. No. 5,311,194 (May 10, 1994), which discloses a GPS precision aircraft approach and landing system wherein the transmitted correction data is modulated on an L-band carrier or “pseudolite” signal that is synchronized with GPS time by way of a GPS timing signal. Both of the mentioned '212 and '194 U.S. patents are incorporated by reference.
For greatest accuracy and safety, a ground station or aircraft carrier in a DGPS landing system deploys so-called reference antennas that are configured to respond optimally to the right-hand circularly polarized (RHCP) signals from the GPS satellites, and to supply the signals to GPS receivers and correction data generating equipment at the station or aboard the carrier. Due to the complex physical and electromagnetic environment of an aircraft carrier, as many as three or more precision GPS reference antennas may need to be installed at determined locations on the carrier so that accurate position correction and other critical data can be determined and transmitted to approaching aircraft or other landing platforms.
To avoid position offset errors, each reference antenna must have a well-defined phase center, and introduce known carrier phase and code phase delays into signals arriving at any given angle of the antenna's reception pattern. The phase center of a GPS receiving antenna is defined as the precise point whose position is being measured in response to GPS signals incident on the antenna. The location of the phase center may vary with the direction of arrival of a given GPS signal mainly as a function of satellite elevation, while azimuth effects may be introduced locally by the environment around the antenna. Thus, it will be understood that ignoring variations in the phase center of a GPS receiving antenna can lead to serious errors in position measurement. See, e.g., G. L. Mader, GPS Antenna Calibration at the National Geodetic Survey, at −http://www.ngs.noaa.gov/ANTCAL/images/summary.html=; F. Czopek, et al., Calibrating Antenna Phase Centers, GPS World (May 2002); and A. Boussaad, et al., A Tale of Two Methods, GPS World (February 2005).
Carrier phase delay is a delay induced on a received GPS signal at its carrier frequency, while code phase delay is a delay induced on the signal over the signal's bandwidth. An ideal GPS right-hand circular polarization reference antenna should have a carrier phase progression of one degree of carrier phase delay per degree of azimuth arrival angle, and remain constant relative to elevation at any given azimuth angle. The antenna should also have a constant code phase delay with angle. Accordingly, ground and ship based reference antennas for DGPS assisted aircraft approach and landing guidance must perform within strict electrical tolerances.
As mentioned, GPS reference antennas are subject to unintentional (lateral and sea multipath) and intentional (jamming) interference which can adversely affect approach and landing guidance. Conventional adaptive array techniques may be employed to form a null in the reception pattern of a given antenna array at an angle that coincides with that of an interfering signal. See, e.g., R. A. Monzingo, et al., Introduction to Adaptive Arrays, Scitech Publishing, Inc. (2004), at pages 6-8 and 260-66; and B. Pattan, Robust Modulation Methods & Smart Antennas in Wireless Communications, Prentice Hall PTR (2000), at pages 241-50. In DGPS applications, the known techniques can act to degrade the overall precision of the system, however. For example, the array's code (group) phase delay and carrier phase delay may be modulated independently for each satellite signal when certain offset elements of a GPS antenna array are weighted and combined. Also, if the array dimensions are too small, it becomes difficult to break the null sharply; that is, the reception pattern stays attenuated over a relatively large angle away from the desired angle of the null. Thus, authentic GPS signals arriving at angles near but not identical to that of the interfering signal go undetected. A reference antenna that suppresses interference without degrading system performance for GPS signals arriving in directions other than that of the interfering signal(s), is therefore highly desirable.
U.S. Pat. No. 5,534,882 (Jul. 9, 1996), incorporated by reference, discloses a GPS antenna comprised of a number of radiator or antenna units stacked one above the other along a vertical support or mast. Each element array includes four dipoles supported in a circular array about the mast, wherein the feed points of the dipoles lie in a horizontal plane and are spaced circumferentially by 90 degrees from one another. Each dipole of a given antenna unit is oriented broadside to the radial direction, and its feed point is aligned vertically with respect to the feed points of corresponding dipoles of the other units. The arms of each dipole are also inclined at a 45 degree angle with respect to the horizontal.
The dipoles of each antenna unit have their feed points coupled with transmission lines that introduce certain progressive phase delays to the dipoles. Also, the dipoles of units above and below a center or reference unit on the mast, have associated phase delays that are offset by, e.g., minus 90 and plus 90 degrees, respectively, from the phase delays associated with the corresponding dipoles of the reference unit. Signals from all the transmission lines are combined to produce an overall reception pattern for the antenna that is optimally responsive to right hand circularly polarized GPS signals arriving from slightly above the horizon (zero degrees) to zenith (90 degrees) in elevation, but which suppresses all signals arriving at or below the horizon, e.g., lateral and ground or sea multipath reflections.
U.S. Pat. No. 6,201,510, also incorporated by reference, discloses a self-contained, four dipole antenna unit with a progressive phase omni-directional (PPO) network that introduces phase delays that increase in 90 degree increments for each dipole. The patent also discloses a GPS antenna including (i) one of the units disposed centrally along the length a vertical antenna mast, (ii) an upper set of ten of the units stacked on the mast above the central unit, and (iii) a lower set of ten of the units stacked on the mast below the central unit. All of the units are spaced from one another along the mast by approximately one-half the operating wavelength. Significantly, the antenna has a unitary phase center for avoiding errors in position determinations, as well as a uniform group delay.
The PPO network associated with the central antenna unit is connected via a transmission line (coaxial cable) to a signal combiner. The unit disposed immediately above the central unit, i.e., the first of the upper ten units, and every odd numbered unit beyond in the upward direction, are also connected by transmission lines to the signal combiner. Further, the unit disposed immediately below the central unit, i.e., the first of the lower ten units, and every odd numbered unit beyond in the downward direction, are connected by transmission lines to the signal combiner. Thus, 11 of the PPO networks (that of the central antenna unit, five of the upper set of units and five of the lower set) are connected to the signal combiner via transmission lines all of which are preferably of equal length. The remaining PPO networks of the upper and the lower sets of units are not connected to the combiner and are simply terminated. The GPS antenna of the '510 patent has a right-hand circularly polarized GPS reception pattern with uniform gain from slightly above the horizon to zenith in elevation, and a sharp cutoff at and below the horizon. As mentioned, the antenna has a unitary phase center as well as uniform group delay.
A compact GPS aircraft antenna whose reception pattern may be adapted when necessary to null or suppress jamming signals arriving from a given azimuth direction, is disclosed in U.S. Pat. No. 6,618,016 (Sep. 9, 2003) which is incorporated by reference. The antenna is in the form of a rectangular cavity about five inches square by one and one-half inches high, with a sheet metal lower surface, conductive peripheral side and interior partition walls, and an upper surface including a printed circuit board. Four slot elements are formed in the upper surface and extend radially from a central vertical axis at 90 degree intervals in the plane of the upper surface. In addition, four bent monopole elements project above the upper surface in a circular array, wherein each monopole element is disposed between a pair of the slot elements and faces broadside to the radial direction.
Each of the slot and the monopole elements has an associated feed point that connects with a corresponding input port of a coupling assembly. The four input ports associated with the slot elements of the antenna connect directly with four output ports of the coupling assembly, and the four input ports associated with the monopole elements connect with a beam forming network contained in the coupling assembly. The beam forming network provides an additional four output ports of the coupling assembly. As disclosed, the antenna of the '016 patent is capable of forming eight different reception patterns depending on which output port of the coupling assembly is processed. A primary beam pattern available at one of the output ports provides omnidirectional (360 degree azimuth) RHCP hemispherical (zero to 90 degree elevation) coverage. The remaining seven patterns including a clover leaf and slot element patterns at four different angular positions, are also available at the output ports and may be combined to form nulls in the antenna's primary beam pattern where and when needed.
Another airborne GPS antenna suitable for anti-jam adaptive processing is disclosed in U.S. Pat. No. 6,819,291 (Nov. 16, 2004), also incorporated by reference. The antenna of the '291 patent features eight monopole elements mounted in a circular configuration above a ground plane with equal circumferential spacing of 45 degrees from one another. Including a protective radome, the overall dimensions of the antenna are about 3.75 inches in diameter by less than two inches height. The antenna includes an orthogonal excitation or coupling network having eight element ports each of which connects to a different monopole element via a transmission line, and eight mode ports (numbered I to VIII in the patent) each representing a different reception pattern for the antenna with respect to incident signals arriving at various angles.
Mode I provides a 45 degree counter clockwise (CCW) progressive phase (PP) coupling of the monopole elements to yield a first omnidirectional RHCP hemispherical or “primary” reception pattern for the antenna. Mode II provides a 45 degree clockwise (CW) PP coupling of the monopole elements to yield a second omnidirectional LHCP hemispherical pattern. Modes III and IV provide 90 degree CCW and CW PP coupling, respectively. Modes V and VI provide 135 degree CCW and CW coupling, respectively. Mode VII provides 180 PP coupling to yield an eight lobe reception pattern for the antenna, and mode VIII provides same phase coupling for all the monopole elements to obtain a uniform phase omnidirectional pattern. Like the antenna of the '016 patent, the antenna of the '291 patent together with the disclosed excitation network can obtain a primary reception pattern (mode 1) allowing omnidirectional RHCP hemispherical coverage in elevation, and the remaining seven or “auxiliary” modes may be combined to effect appropriate nulls in the antenna's primary reception pattern when jamming signals are detected, using known adaptive array processing techniques.
U.S. Pat. No. 7,417,597 (Aug. 26, 2008), incorporated by reference, discloses a GPS antenna having a vertical mast, with 13 four-dipole element arrays stacked vertically on the mast. Each element array is identical or similar to the element arrays disclosed in the mentioned '510 U.S. patent, and includes a progressive phase omnidirectional (PPO) network coupled between a common feed port and each of the four dipoles of the array. Counting from the lowermost element array on the mast, the second, fourth, sixth, seventh, eighth, tenth and twelfth arrays are designated as primary radiator units, and the remaining arrays are designated as auxiliary radiator units. Outputs from the feed ports of the primary radiator units are individually weighted and input to a first signal combiner, and outputs from the feed ports of the auxiliary radiator units are individually weighted and input to a second signal combiner. Outputs from the two signal combiners are then subtractively combined by an adjustable signal combiner under the control of an adaptive control system.
The control system is operative to steer a single, omnidirectional null in the antenna's reception pattern so as to suppress an interfering signal arriving at an elevation angle between minus eight and plus five degrees relative to the horizon. In the event multiple interfering signals are detected at different elevation angles, the null may be steered to a determined angle between the interferers, but such a solution is not effective to eliminate all potential errors attributable to the interferers.
The patented GPS ground antennas described above are in the form of vertical collinear arrays of individual antenna units that are phased to produce primary reception patterns that are hemispherical above the horizon, and which cut off at and below the horizon with low side lobes. The phasing inherently suppresses ground and sea multipath. Lateral multipath is suppressed when the GPS satellites are at low elevation angles due to the right hand circular polarization of the antennas. That is, reflections of GPS signals off of ship structures including tall aircraft, as well as off of the water, will acquire a left hand circular polarization which is orthogonal to the right hand circular polarization of the primary reception pattern of the antennas. The antennas are also constructed so as to have point group and phase delay centers that closely coincide with the group and the phase delay centers of the auxiliary radiator units of the antennas so that the centers are preserved during adaptive processing, and precision approach and landing guidance is maintained in the presence of interference and jamming.
The GPS antenna of the mentioned ‘597 U.S. patent enables a single null to be steered in a low elevation plane of the antenna's reception pattern for jamming rejection. More complex threats in the form of multiple jammers each at differing elevation angles of arrival may exist, however. Such threats require an ability to induce discrete nulls in the antenna's pattern at multiple angles of azimuth and elevation. Thus, there is a need for a GPS antenna that will enable discrete azimuthal nulls to be steered to suppress interfering signals arriving near the horizon but at different elevation angles. There is also a need for a GPS antenna that will enable multiple nulls to be induced in its reception pattern at any elevation angle above the horizon.